World's Heaviest Bony Fish Discovered?

John Roachfor National Geographic News

May 13, 2003

In fishing circles, talk of the big ones always garners an audience. The bigger the fish, the more awe it inspires. A crowd may soon gather around Tierney Thys, a marine biologist with the Monterey, California-based Sea Studios Foundation, and Chuck Farwell from the Monterey Bay Aquarium.

As part of Thys's study of the little-known ocean sunfish (Mola mola), a behemoth that can grow more than 4.2 meters (14 feet) long from dorsal fin tip to anal fin tip and 3 meters (10 feet) in horizontal length, she has uncovered a fish that may be a new record setter for the world's heaviest bony fish.

The fish in question is an ocean sunfish that was caught off the coast of Kamogawa, Japan, in 1996 in set nets owned and operated by the Kamogawa Fisheries Cooperative Association. Members of Kamogawa SeaWorld measured the fish to be 2.7 meters (8.9 feet) long and say it weighed 2.3 metric tons (5,071 pounds).

"It is not the longest, but it may be the heaviest sunfish that's actually been measured on a reliable scale," said Thys, who is studying the ocean sunfish in a satellite tagging project supported in part by the National Geographic Society's Committee for Research and Exploration.

The project aims to gather the first-ever baseline biological data on the ocean sunfish, offering insights to how they migrate, whether regional populations interact, and where they spawn.

Thys and her colleagues are concerned that the big fish are declining due to their accidental catch by drift net and longline fisheries. The tagging program, they hope, will lead to conservation and appreciation of the fish before they are lost forever.

Thys and her colleagues are currently analyzing the data they have received from satellite tags attached to nine ocean sunfish and plan to publish their findings within a few months in a scientific journal. Meanwhile, they are tracking down just how big the biggest of these fish can get.

Current Record

The current record for the heaviest sunfish in The Guinness Book of Animal Records belongs to a Mola mola that was struck on September 18, 1908 by the Australian steamship SS Fiona about 40 miles (65 kilometers) from Sydney. The fish was towed to port where it was measured and apparently weighed.

According to accounts of the incident published in the March 1928 issue of Scientific Monthly and the December 10, 1910 issue of The Wide World Magazine, the fish measured 3.1 meters (10 feet) in horizontal length, 4.26 meters (14 feet) in vertical length, and weighed 2 tons 4 hundredweight, which converts to 4,927 pounds.

But there is some doubt as to how this particular fish was weighed. Among the skeptics is Julian Pepperell, a marine biologist and fisheries historian with Pepperell Research and Consulting in Noosaville, Australia.

Pepperell said that according to an account by David Stead, the local scientist at the time who would likely have been the most familiar with SS Fiona incident, the mola was never officially weighed, only measured. The weight, which he says is plausible, was probably a guesstimate.